insidious

From Middle French insidieux, from Latin īnsidiōsus (“cunning, artful, deceitful”), from īnsidiae (“a lying in wait, an ambush, artifice, stratagem”) + -ōsus, from īnsideō (“to sit in or on”), from in (“in, on”) + sedeō (“to sit”).

• IPA^(key): /ɪnˈsɪdi.əs/

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insidious (comparative more insidious, superlative most insidious)

1. Producing harm in a stealthy, often gradual, manner.
   • 1847, George Lippard, The Quaker City: or, The monks of Monk-Hall:

     Strong and vigorous man as he looks, Livingstone has been for years the victim of a secret and insidious disease.

   • 1997, Matthew Wood, The book of herbal wisdom: using plants as medicine:

     At some point in time they may become the source of an insidious cancer.

   • 2007, Sharon Weinstein, Ada Lawrence Plumer, Principles and practice of intravenous therapy:

     The nurse always must be alert to signs of slow leak or insidious infiltration.

   • 2023 March 8, Gareth Dennis, “The Reshaping of things to come...”, in RAIL, number 978, page 49:

     The impact on rural communities of rail closures was acute, but I would argue that the worst outcome it created was the long-term diminishment of suburban capacity outside London, which has had a far more insidious effect on rail usage nationally.

2. Intending to entrap; alluring but harmful.
   • 1841 February–November, Charles Dickens, “Barnaby Rudge”, in Master Humphrey’s Clock, volume III, London: Chapman & Hall, […], →OCLC, chapter 48, page 215:

     Gashford slid his cold insidious palm into his master's grasp, and so, hand in hand, and followed still by Barnaby and by his mother too, they mingled with the concourse.

   • 1850, Nathaniel Hawthorne, chapter 5, in The Scarlet Letter, a Romance, Boston, Mass.: Ticknor, Reed, and Fields, →OCLC:

     The insidious whispers of the bad angel.

   • 1948, D.V. Chitaley (editor or publisher), All India Reporter, volume 3, page 341:

     All these facts clearly appear to me now to establish that the sanctioned scheme was a part of a bigger and […] more insidious scheme which was to hoodwink the creditors and to firmly establish and consolidate the position […]

   • 1969, Dorothy Brewster, John Angus Burrell, Dead reckonings in fiction:

     The atmosphere of this insidious city comes out to meet him the moment he touches the European shore; for in London he meets Maria Gostrey just over from France.

   • 1983, James C. H. Shen, “Rejoining the Government”, in Robert Myers, editor, The U.S. & Free China: How the U.S. Sold Out Its Ally‎, Washington, D.C.: Acropolis Books Ltd., →ISBN, →LCCN, →OCLC, page 28:

     They all sought the President's views on the world situation in general and the Asian situation in particular. Without mincing words he would comment on his favorite theme, namely, the insidious scheme of the international Communists to conquer the free world.

   • 2005, Anita Desai, Voices in the City, page 189:

     This seemed to her the worst defilement into which this insidious city had cheated her and in her agitation, she nearly ran into the latrine, […]

   • 2007, Joseph Epstein, Narcissus Leaves the Pool, page 171:

     This is the insidious way sports entrap you: you follow a player, which commits you to his team. You begin to acquire scraps of utterly useless information about teammates, managers, owners, trainers, agents, lawyers.

   Hansel and Gretel were lured by the witch’s insidious gingerbread house.

3. (nonstandard) Treacherous.
   • 1858, Phineas Camp Headley, The life of the Empress Josephine: first wife of Napoleon:

     But with whom do you contract that alliance? With the natural enemy of France — that insidious house of Austria — which detests our country from feeling, system, and necessity.

   • 1912, Ralph Straus, The prison without a wall:

     ‘Believe me,’ he shouted, ‘these insidious folk talk dangerous nonsense. I hear they are spouting out their ridiculous platitudes not five miles from this park in which we are standing…’

   The battle was lost due to the actions of insidious defectors.

• insidiously
• insidiousness

• “insidious”, in Webster’s Revised Unabridged Dictionary, Springfield, Mass.: G. & C. Merriam, 1913, →OCLC.
• “insidious”, in The Century Dictionary […], New York, N.Y.: The Century Co., 1911, →OCLC.
• “insidious”, in Dictionary.com Unabridged, Dictionary.com, LLC, 1995–present.